1. Field of the Invention
The present invention relates to a camera with an AF (Auto Focus) mechanism using a phase-difference method, such as an SRL (single-reflex lens) camera. In particular, it relates to a control for a series of line sensors.
2. Description of the Related Art
An SRL-type camera is equipped with an AF mechanism based on a phase-difference method, which has an optical image-forming system and a sensor module that is constructed from a focus-detection device, such as an IC chip. The optical image-forming system has a separator lens and a condenser lens, whereas the focus-detection device is equipped with a series of line sensors that are arrayed within a projection area of the optical system. Each line sensor has a plurality of pairs of photodiodes.
In an AF sensor, an optical system divides a ray bundle from an object into two ray bundles to project a pair of images onto the pairs of line-sensors. Each line sensor outputs image-pixel signals by photoelectric conversion, and a difference between the positions of the two images, namely a phase difference, is detected. The difference represents an out-of-focus magnitude. Whether or not an object is in focus can be determined by the detected out-of-focus magnitude. When the object is out of focus, a length of movement and shift in the direction of the focusing lens are determined. Then, the focusing lens is driven.
Generally, a line sensor is an electric charge storage/accumulation-type sensor, and an accumulation period is adjusted on the basis of a monitor sensor that is arrayed alongside the line sensor. For example, U.S. Pat. No. 7,493,034, U.S. Pat. No. 6,937,818 and U.S. Pat. No. 4,876,603 each discloses the adjustment of an accumulation of electric charges using a monitor sensor. A monitor sensor, which has a photo-electric converter such as photodiodes, continuously detects the intensity of light and outputs monitor signals in order to prevent the target line sensor from receiving a light quantity in excess of a dynamic range, which would cause electric charges to overflow from the line sensor. The amount of light that each line sensor receives varies according to the brightness distribution of an object. Therefore, a charge-accumulation period is controlled independently for each line sensor.
A monitor signal level is compared with a predetermined threshold level during the accumulation of electric charges. When a monitor signal exceeds a predetermined threshold value, a corresponding line sensor stops the accumulation of electric charges by opening a transfer gate, and accumulated electric charges are temporarily stored in a memory (e.g., a capacitor) of the line sensor. After the accumulation of electric charges in all of the line sensors is completed, a series of image-pixel signals that correspond to image signals of an object are output from the line sensors. At this time, the series of image-pixel signals are subjected to a noise reduction process and converted to digital signals. The output digital image signals are used for calculating an out-of-focus parameter. A focusing lens is then driven by an amount equal to the value of the out-of-focus parameter. The output of image signals is repeatedly continued until an object image is brought into focus.
While reading accumulated electric charges from a line sensor, electric charges that are generated in photodiodes by incident light are cleared away so that no new electric charges accumulate in the line sensor. Namely, an accumulation of electric charges and the reading of accumulated electric charges are carried out independently, and in order. Therefore, when the length of a period for reading accumulated electric charges increases due to the brightness of a target object (e.g., a dark object), it takes a long time to focus on the object.